I have both the MBP 16" and MBA 15", and have been putting it through some tests to see which one I will keep.
The Macbook Air 15" screen is great. IF you haven't really seen the 16" MBP screen, you would be happy. But when you compare the 2 side by side, you WILL see the difference.
It's not noticeable when the images are bright. Colors pop and there's little conceivable difference from the MBP. The MBA looks great here. Text, at least to me, both look great. I give a slightest edge to the MBP, but MBA text is sharp and clear.
You will notice the difference when you view images/videos with dark scenes. Notice how much more contrast you get from the MBP. The blacks are washed out in the MBA.
When you watch HDR content, you will notice that the MBP is brighter, although not by as much as you would think. Some highlights (sun, street lights) will appear brighter. But the MBA gets plenty bright, even when capped out at 500nits. Certain videos can make the MBP get super bright , but for most content, it's not that noticeable.
Promotion is also noticeable, but in reality, it doesn't make much of a difference as you are viewing static content most of the time. Even watching videos/movies you won't notice any motion difference. It's only in strolling and moving content around the screen.